Method of making optical lenses



June 15, 1948.

A. W. KINGSTON METHOD QF MAKING OPTICAL LENsEs 'Filed July 25, 1940 Il' lulII mil/lll,

mm m' Mg INVENTOR:

ARTHUR h/, l/Ncsrou wf @m l .Mlm

Patented June l5, 1948 OFFICE METHOD OF MAKING OPTICAL LENSES Arthur William Kingston, Denham, near Slough, England Application July 23, 1940, Serial No. 346,927 In Great Britain February 23, 1934 4 Claims. 1

This invention relates to a method of manufacturing optical lenses and similar devicesrfor use in instruments of precision. This application is a continuation-impart of .my application Serial No. 84,170 filed June 8, 1936, which issued into Patent 2,314,838, my application Serial No. 84,170being a continuation-impart of my application Serial No. 735,592 flied July 17, 1934, now abandoned.

According to the present invention a method of manufacturing optical devices is characterised in that a transparent thermoplastic material is given by a single moulding operation the shape and highly polished surface necessary for such useby beingpressed between dies of known type but having surfaces polished to a degree not heretofore disclosed or practised, such pressing being done at a temperature sufliciently low not to injure such highly polished surfaces of the dies.

yObjects of the invention herein claimed comvprise the'provision of improved methods of forming optical lenses, and other optical devices, of the character in which the optical qualities, such as the dioptric curves of the surfaces of lenses, must be of a high degree of precision, and the provision of any improved sub-processes and process steps which may be involved therein.

By ,optical devices I mean lenses, prisms, mirrors and like devices used to refract or reect light which are suitable for use in instruments of precision such as cameras, field and opera glasses, telescopes,.microscopes, projectors, and the like, spectacle and pince-nez lenses and the like being other examples of the precision lenses formed by the invention, but I do not include lamp glasses, rough lenses or reflectors such as are commonly used in electric torches or other hand lamps where the optical perfection is of small importance and which are therefore not pnovided with optically polished surfaces. By optically polished I mean having such a degree of polish as is not less than that found upon the surface of glass lenses used in instruments of precision today. The higher the polish the better will be the resulting lens.

'By moulding I mean any process where by the required shape in three dimensions is imparted to the raw material by dies exerting a pressure.

By transparent material I mean one through which light will pass to a degree comparable with that of ordinary glass.

Transparent materials which are sumciently plastic' to be moulded at a temperature between C; to 150 C. which temperature is suiflciently low as not to injure the optical polish on a diesurface are commercially available today. Such material may be a celluluose derivative or a synthetic resin, and these materials can readily be moulded at the required temperature and there is no distortion4 in the structure as the material cools. Examples of these materials are a polymerised styrol resin and an acrylic acid resin which are sold respectively under the names of Trolitul" and Perspex. It is noted that the temperatures at which such materials become plastic, as indicated by the above examples of materials and temperatures, are to be compared with the much higher temperatures at which glass fuses, and which would injure the optical polish of die-surfaces, if an attempt were made to mould accurate lenses, etc., therefrom. Optical devices made of these materials are superior to those made of glass because their transmission of the violet end of the spectrum is high. For example, whereas ordinary crown glass passes only 20% of ultra violet light Perspex passes '70%.

Specic embodiments of devices produced by, and means for practising the present invention will now be described with reference to the accompanying drawings of which- Figure 1 shows a combined lens and mount;

Figure 2 shows a modified form of combined lens and mount;

Figure 3 is a diagrammatic View of a press for moulding the lenses;

Figures 4-7 are side elevations in section of various forms of lenses;

Figure 8 is a side elevation of yet a further modification partly in section taken along the line 9-9 of Figure 9;

Figure 9 is a front elevation of the modification shown in Figure 8;

Figures 10 and 11 are respectively a front and side elevation of yet a further modification;

Figure l2 is a front elevation of a further modiiication;

Figures 13 and 14 are respectively a side elevation in section and a perspective view of a moulded prism;

Figure 15 is a side elevation in section of a mirror, and

Figure 16 is a sectional elevation of a combined eye-piece and lens.

As' shown in Figure 1, a moulded structure of transparent organic material comprises a lens 20 and a cylindrical mount 2| surrounding the lens 20 and formed integrally therewith.. The portion 3 of the surface of the dies that form the lens is provided with an optical polish but the portion that forms the mount is not polished and may be provided with a milled or other roughened surface so as to facilitate subsequent staining of the same to produce opacity. The combined lens and mount maj/'readily bev introduced into a holder-22 and moved backwards and forwards relatively to the holder for the purpose of focussing. It will be seen that the mount, in addition, constitutes.y

a protecting rim for the lens.

In order to facilitate the movement'of thecorn bined lens and mount for the purposeof focus-U sing, the mount may have a screw-thread 23 moulded integrally therewith as shown in Figure 2. The screw-thread 23 is adaptedzto coactfwith.y

a groove in the holder, and a collar 24i`s-prow vided to facilitate rotation of the mountf and to act as a stop.

A press for mouldingv the lenses is shown in Figure 3. The press which is secured to a support 25 comprise-:sfdiest` and 21 vwhich are respectively secured to rods 23 andi29.- The rod 28 is held in a chuck 33 which is mounted on the lower end y of a rod 3i, and the -rod 3l is adapted to slide yup anddown in4 a sleevev 32` which is secured to the support 25. A lever 33 which is pivotally secured to a bracket 34 engages -a pin 35 secured to the upper end of the rod 3 I.

The lower end of the `rod 29 is engaged by a lever 36 which is mounted in a similar manner to the lever 33 `so that the rod 29 togetherwith the die 27A can be raised when# desired. A collar 3l is provided on the rod 29 which engages a plate 3d and-acts as a stop to prevent the die. 2'l'from being lowered beyond apredeterminedpoint.

A mould casing 39 which coi-operates withthe dies is secured to the supportZE, and comprises a sleeve 4@ which is inregisterA with the dies-and which is surrounded by a jacket 4|forf steamer cooling water. Inlet pipes 42 and 43 for steam and cooling water respectively arel connected to the casing and are each provided with a valve ld and 45 -for controllingthe flow-of luidinto the jacket, An exhaust pipe-.46--for. steam and water is provided on the side ofthe .jacket/.re-y

mote from the inlet pipes.H f

A jacket 30, similar-tothe jacket-4l surrounding the die 2l; surrounds'the die v2li` and comprises inlet pipes 6l and 62 for steamfand cooling water respectively, andY anfexhaust pipe63. The

inlet pipes are provided with valvesr6,4.and .65lfor controlling the iiow of fluidvin the jacket.

The press is operated as follows: The resinvor like material of which-the lens is to beA moulded is cutfinto a number of pellets which will tinside the sleeve dll, The size of each pellet. is -accurately calculated so that it contains suflicient material for the particular lens which is to be made, without having undue waste material... The steam-valves 44 `and 64 are opened-sothat steam iiows through the jackets 4l and 6U and the .dies

25 and Z1 are heated. A pellet 41 is nextplaced.

reached the correct temperaturev forlmoulding,

this method of employing only a light initial pres- (l5 The diez26 is then sure obviates the formation of cracks and faults which occur if the full pressure only is applied.

After the light pressure has been applied for a few seconds the full pressure is applied and maintained after the pellet has been moulded until it has been cooled by circulating cooling water through the .jacket 4I in place of steam. The relative movement of the dies towards each other is, obviously, stopped when the desired thickness of the lens or other object moulded has been attained, this stoppage being accomplishedin any usual way. This follow-up pressure: ensures that the moulded pellet is always in contact with the `dies until it is cool whereby warping due to the shrinkage caused by the drop in temperature is prevented and the surfaces of the" moulded pellet conform accurately to the surfaces of the dies. The initial pressure is preff! erably of the order of 10% to 20% of the iinal pressurafor example, a lens of 2 square .inches area. is 4moulded-with an initialtotal` pressure of from the middle 'towards the edges, i. e. outwardly between .the die surfaces. For example, with-a die having a concave spherical surface the' pelletv` is given a convex spherical surfaceof.slightly-less" radiusv of curvature, vIt vwill be seen that' if the" pellethad a'latsurface or one having a greater-y radius of curvature than' the die, the edges 'of the. die, would engagethe pellet rst and fair wouldbecome trapped in the' space between the facesofthe pellet andthe die therebyfprevent ingthe die from making proper contact withthe pellet and consequently impairing` theaccuracyv ofthe moulding. The present process ensures that allthe air is driven outfromthe space rbc'- tweenthelsurfaces of the die andpellet andensuresthat the .die makes proper contact. withthepellet. It is tobe noted, however; that in the moulding process a certain amount of iiash" is lformed` on the peripheral edge'of the `lens due to theflow outwardlyl of the heatedppellet under moulding pressure. Materials suchas Perspex are usually cast into sheets but they vcould rea'dilybe originally cast into the .preformed-pellets above described.

When the. lens has .reached a temperatureof i the order. of.20 C.' the .die .26 `is. raised bymeans of. the 4lever 33 and the' nished lens is ejectedfrom the cylinder by raising the dieZT by meansof the lever 36.- The dies 26 and 2'T:are-ma'de"of a steel known commercially as a high'. chrome Steelandthey are provided withya degree of polish .whichis not less than that-found upon glass lenses which aretoday usediininstrumentsf The dies are o1- complementaryof. precision. shapetothe lenssurfaces so that themoulded lenshasthe requiredshape and polish imparted' to it during the moulding operation andis ready for. usein an instrument of precision when-it` leavesthe mould, without any subsequent-grinding or polishing.

It will be noted that thecooled; lens after'subv jection to the above 'described-treatmentis `free from internal stresses such as mightsubsequently` ltween the lens and the holder. vFigure 3, the upper half of the sleeve 40 is provided with recesses I which form the projeccause 7Vtlfiegllens..to .warp The" surfaces o of thes 1enses",or..fother devices` as j stated are optically truajand'. smooth, andthe lenses produced. for` use inf. instruments. of, yprecision such asxtele=i scopes, microscopes, spectacles, etc., compare; favourably inY opticalv characteristics with. well" ground. glass`lensesinade for the same purposes. Also as `stated..theflenses produced are, foriall practical purposes, unbreakable; they .are non'- plastic at -all ordinary temperatures, and when' made ,Tof A,thespecifi'c '..materials above referred' to are free from discoloration or warping.

Itwill.be .appreciatedl that any. optical devices for use in instruments of precision, e. g. lenses, prisms or mirrors=can readily be formed in a single molding operation and that they may be formed integrally with protecting rims or mounts as desired, a feature which haslnotf. heretofore been possible owing` to they relative movement which has to take place between the grinding tool.,.and"the optical deviceproduced thereby;-

- projects outwardly beyond each of the optical faces so that the lens may be placed face downwardly, on a substantially flat surface with only the rim touching the surface so that the optical faces are not scratched. The rim is preferably provided with one or more projections 50 which may be arranged to co-operate with grooves in the lens-holder to prevent relative rotation be- As shown in tions 50.

Figures 5, 6 and 7 show respectively a planoconvex, plano-concave and a concave-convex lens formed integrally with a rim 49 which serves to protect at least one of the optical surfaces. Figures 8 and 9 show a double convex lens of rectangular form. The optical surfaces 52 and 53 are spherical so that at each corner of the surface 53 there is provided a flat portion 60 which serves as a seating for a holder. The surface 52 is provided with a protecting rim 49 and one of the edges is formed with projections 50 as above described, which projections are shown as being nearer one corner of the lens than the other so as to ensure that the lens is -correctly placed in the mount which will have co-operating recesses.

Instead of forming a continuous protecting rim completely surrounding the optical surface, a number of outwardly-directed excrescences may be provided for the same purpose. As shown in Figures 10 and 11 a. lens similar to that shown in Figure 4 may be provided with four equally spaced outwardly-directed excrescences 54 in place of the outwardly-projecting rim 4K9. It will be seen that these excrescences serve equally well to prevent the optical surfaces of the lens from being scratched and at the same time reduce the amount of plastic material used. The excrescences 54 on one side of the lens may be adapted to register with recesses in the mount and mal7 be arranged as shown in Figure 12, with a different space between each pair of excrescences s softliattheilensfcaw only beiplacedeinlthe mmmtl in thecorrectrnannere bemoulddfby4 suitably adapting thef dies'fgso ithatt` a protecting-snm dl'lsurrounds-"each: optical.y s'urul fafcei'nElarid;J as'shown \in"Fig'ure- 15 j a mirror may li'ke'wiseffbe-y providedLA with f a protecting-1- `rimV 49,:v the reiiectingfsurface of 'the-2 mirror being-1 consti -V tuted=-by a layer ofsilver-'upon'an optical surface. Thefrim 'which surrounds theliopticalfsurface.A mayereadilyl 'befenlarged to provide "la Amount :for the-fdevice an dftlievmountv mayA beso-shaped thatl i thefwheleimculdirig"constitutes a self-:contained y articl'eelfAsshown in Figure l16,4fan' eye;pi-ece1forfieldPglasses-comprisesf a -double-`1concave'lens 5-, a screw-threaded?mountliil; anda tapered por==vv tion-11 58 f Tmay'- be-moulded aso a.1-uni-ta.ry-articles`v v Th'eA mount f 51 and theiitapered fporti'onl58'cona stitutes a protecting rim for the lens 56.

I claimlz 1. "-.A' -.methodoof fforming. optical flensesf of high f. precision" 'havinga desired focal length';V 'from fanorganic,I` transparent thermoaplas'tic` resin'V which comprises:A placing-a fsolidi', blankofv` organic,x transparent thermo-plastic 'resin of lbut-. slightly l greater-thickness than that @desired in1 the 4iin-1I ished lens betweensurfaces =ofI opposing dies-,lsaid 1 surfacesl `comprising.-the reverse -of substantially i opticallyaperiect surfaces desired-f inithe finished lens; lheating; the: dies :and thermoeplasticriresin to 1 a. .temperature at f-.which' therzresinri. becomes:

suiciently plastic to flow only under pressure, bringing about relative movement of the dies to press the thermo-plastic resin with sufficient pressure to cause the resin to flow outwardly from the centre of the die surfaces and to conform to the contours of the die surfaces; stopping the movement of the dies to obtain a lens of desired thickness and cooling the dies and resin to a temperature below that at which the resin sets while maintaining a pressure on the resin, and then removing the formed resin from between the dies.

2. A method of forming lenses adapted for use in instruments of precision which have a desired focal length; from an organic thermo-plastic material, which comprises: placing a solid blank of organic transparent thermo-plastic material of greater thickness than that desired in the finished lens between surfaces of opposing dies, said surfaces comprising the reverse of substantially optically perfect surfaces desired in the finished lens, said dies being so formed as to permit excess thermo-plastic material to flow out from between said dies; heating the dies and thermo-plastic material to a temperature at which the material becomes sufficiently plastic to flow only under pressure; moving one of said dies toward the other to press th'e thermo-plastic material with suflicient pressure to cause the material to conform to the contours of the die surfaces and to flow outwardly between the die surfaces; stopping the movement of the dies toward each other to obtain a lens of desired thickness and cooling the dies and material to a temperature below that at which it can be bent and worked while maintaining a pressure on the material, and then removing the formed material from between the dies.

3. A method of forming spectacle lens which have a desired focal length, from an organic, transparent thermo-plastic resin which comprises: placing a solid blank of organic, transparent thermo-plastic resin of greater thickness than that desired in the finished lens between surfaces of opposing dies, said surfaces comprising the reverse of substantially optically perfect surfaces desired in the finished lens, said dies being so formed as to permit excess thermo-plastic resin to iiow out from between said dies; heating the dies and thermo-plastic resin to a temperature at which the resin becomes sufficiently plastice to flow only under pressure, moving one of said dies toward th'e other to press the thermoplastic resin with sufficient pressure to cause the resin to conform to the contours of the die surfaces and to flow outwardly between the die surfaces; stopping the movement of the dies toward each other to obtain a lens of desired thickness and cooling the dies and resin to a temperature below that rat which it can be bent and Worked while maintaining a pressure on the resin, and then removing the formed resin from between the dies.

4. A method of forming spectacle lenses which have a desired focal length, from an organic thermo-plastic material, which comprises: placing a solid blank of organic, transparent thermoplastic material of greater thickness than that desired in the finished lens between surfaces of opposing dies, said surfaces being highly polished and of a hardness as great as that of high chrome steel, and comprising the reverse of substantially optically perfect dioptric curves desired in the nished lens, heating the dies and thermo-plasticy material to a temperature at which the material becomes suillciently plastic to ow only under pressure; moving one of said dies toward the oth'er to press the thermo-plastic material with suiilcient pressure to cause the material to conform to the contours of the die surfaces and to lflow outwardly between the die surfaces; stopping the movement of the dies toward each other to obtain a lens of desired thickness and cooling the dies and material to a temperature below that at which it can be bent and worked while maintaining a pressure on the material, and then removing the formed material from between the dies.

ARTHUR WILLIAM KINGSTON.

REFERENCES CITED The following references are of record in the ile of this patent:

UNITED STATES PATENTS 

